Sub Bottom Out

Hi What Makes A Sub Bottom Out.it Hit Hard Low But When I Turned Up It Bottom Out Why.i Have Two Xd 15 Thats 700 Rms A Piece And 1500 Max And My Amp Is 1000rms A.b Class.i Have Aported Box That 18h,40 Long 20depth And 4 Inch Ported That 16 Inches Long